In electrophotography, an electrostatic charge image is formed on a dielectric surface, typically the surface of the photoconductive recording element. Development of this image is typically achieved by contacting it with a two-component developer comprising a mixture of pigmented resinous particles, known as toner, and magnetically attractable particles, known as carrier. The carrier particles serve as sites against which the non-magnetic toner particles can impinge and thereby acquire a triboelectric charge opposite to that of the electrostatic image. During contact between the electrostatic image and the developer mixture, the toner particles are stripped from the carrier particles to which they had formerly adhered (via triboelectric forces) by the relatively strong electrostatic forces associated with the charge image. In this manner, the toner particles are deposited on the electrostatic image to render it visible.
It is generally known to apply developer compositions of the above type to electrostatic images by means of a magnetic applicator, also known as a magnetic brush, which comprises a cylindrical sleeve of non-magnetic material having a magnetic core positioned therein. The core usually comprises a plurality of parallel magnetic strips arranged around the core surface to present alternating north and south oriented magnetic fields. These fields project radially, through the sleeve, and serve to attract the developer composition to the sleeve outer surface to form what is commonly referred to in the art as a “brush” or “nap.” Both the cylindrical sleeve and the magnetic core may be rotated with respect to each other to cause the developer to advance from a supply sump to a position in which it contacts the electrostatic image to be developed. After development, the toner depleted carrier particles are returned to the sump for toner replenishment.
Conventionally, carrier particles made of soft magnetic materials have been employed to carry and deliver the toner particles to the electrostatic image. U.S. Pat. Nos. 4,546,060, 4,473,029 and 5,376,492, the teachings of which are incorporated herein by reference in their entirety, teach use of hard magnetic materials as carrier particles and also apparatus for development of electrostatic images utilizing such hard magnetic carrier particles. These patents require that the carrier particles comprise a hard magnetic material that when magnetically saturated exhibits a coercivity of at least 300 Oersteds and induced magnetic moment of at least 20 EMU/gm in an applied magnetic field of 1000 Oersteds. The terms “hard” and “soft” when referring to magnetic materials have the generally accepted meaning as indicated on page 18 of Introduction To Magnetic Materials by B. D. Cullity published by Addison-Wesley Publishing Company, 1972. These hard magnetic carrier materials represent a great advance over the use of soft magnetic carrier materials in that the speed of development is remarkably increased with good image development. Speeds as high as four times the maximum speed utilized in the use of soft magnetic carrier particles have been demonstrated.
In the methods taught by the foregoing patents, the developer is moved in the same direction as the electrostatic image to be developed by high-speed rotation of the multi-pole magnetic core within the sleeve, with the developer being disposed on the outer surface of the sleeve. Rapid pole transitions on the sleeve are mechanically resisted by the carrier because of its high coercivity. The nap, also called “tstrings” or “chains”, of carrier (with toner particles disposed on the surface of the carrier particles), rapidly “flips” on the sleeve in order to align with the magnetic field reversals imposed by the rotating magnetic core, and as a result, moves with the toner on the sleeve through the development zone in contact with or close relation to the electrostatic image on a photoconductor. This interaction of the developer with the charge image is referred to as “contact” or “contacting” herein for purposes of convenience. See also, U.S. Pat. No. 4,531,832, the teachings of which are also incorporated herein in their entirety, for further discussion concerning such a process.
The rapid pole transitions, for example as many as 467 per second at the sleeve surface when the magnetic core is rotated at a speed of 2000 revolutions per minute (rpm), create a highly energetic and vigorous movement of developer as it moves through the development zone. This vigorous action constantly recirculates the toner to the sleeve surface and then back to the outside of the nap to provide toner for development. This flipping action thus results in a continuous feed of fresh toner particles to the image. As described in the above-described patents, this method provides high density, high quality images at relatively high development speeds.
U.S. Pat. Nos. 4,666,813 and 5,024,915 teach the use of silica as a surface treatment and that the silica may include a fine powder of anhydrous silicon dioxide (silica) or silicates such as aluminum silicate, sodium silicate, potassium silicate, magnesium silicate and zinc silicate.
U.S. Pat. No. 5,763,130 mentions zinc silicate as a possible surface treatment with a preference given to those containing not less than 85% by weight SiO2.
US Patent Application 2003/0190543A1 describes doping silica with Zn via the hydrolyzing flame process (fumed silica) followed by silicone coating.
US Patent Application 2005/0058924A1 describes a toner for electrophotography in which hydrophobic fine particles obtained by coating fine silica particles with a hydroxide or an oxide of one or more of titanium, tin, zirconium and aluminum in an aqueous system, and further coating surfaces thereof with an alkoxysilane are used as an external additive.
EP 774696B1 describes the preparation of crystalline strontium silicate surface treatment.
U.S. Pat. No. 5,385,798 describes colloidal silicas treated with boric acid or salts such as lithium sodium and other alkalis.
U.S. Pat. No. 5,397,667 describes surface treated silicas. Specifically, this reference describes metallized silica preparation via neutralization of acid groups on R972 with lithium, sodium and potassium hydroxides. The metallized surface is then treated with a long chain alcohol.